arcasses from the ongoing cull of around 2,600 birds, including ducks and geese, are loaded into a container lorry for disposal. Photograph: Jamie McDonald/Getty

The hunt for the source of Britain's fourth outbreak of the highly pathogenic H5N1 bird flu centred last night on an ornamental lake at a Suffolk stately home, where free range turkeys from an infected poultry farm have been mixing with wild birds.

The outbreak at Redgrave park farm near Diss was confirmed yesterday by vets. Last night the cull of more than 5,000 turkeys, 1,000 ducks and 500 geese on the farm was continuing as officials tried to contain the virus and discover its source.

Early indications were that there had been no spread of the disease beyond Redgrave park, a mansion whose grounds have been let out to Gressingham Foods, Britain's largest duck farmers. The company provides birds to some of Britain's most renowned restaurants.

The acting chief vet, Dr Fred Landeg, said there had been no movements of birds off the Redgrave farm since the incubation of the disease started, probably one week ago. "It is too early to speculate how this virus got to these premises, but the initial character of the virus suggests it is of Asian lineage closely related to strains found this summer in the Czech Republic and Germany. It does suggest the possibility of a wild bird source," he said.

It is understood that the turkeys, which are housed at night, have been allowed to roam around the park and mingle with wildfowl which regularly use the lake.

But ornithologists last night said that no wild birds had been found dead in the vicinity and there have been no reports of wild birds dying of bird flu on migration routes to Britain.

Last night the RSPB urged the government not to leap to conclusions. "Jumping to conclusions over the source of bird flu could blind us to courses of action that should be taken," said Dr Mark Avery, its conservation director. Landeg warned that the investigation and culling could take some time because the free range farm has minimal access facilities.

Samples were last night being taken from the slaughtered birds to check how many were infected. More than 10% of a batch of turkeys were found to have died, but none of the geese or ducks

Last night a two-mile (three km) protection zone and a six-mile surveillance zone had been set up around the infected farm to protect breeders. A further restriction zone stopping the movement of poultry now covers most of Norfolk and Suffolk.

The disease would devastate the immense Christmas trade in poultry if it spreads. More than 200m birds are registered in the UK and there are more than 3m turkeys being reared by 90 farmers in the restricted zone around the farm.

Reports that Gressingham Foods, based in Woodbridge, Suffolk, had been buying in poults, or day old chicks, from the Czech Republic - where there had been H5N1 outbreak earlier this year - were also being investigated. "There is no indication yet that this is the case. We will be investigating imports and possible movements," said Landeg.

The company said yesterday on its website that it had installed the latest biosecurity measures at all its sites, including vehicle disinfection systems, the control of staff and a review of policies regarding staff travelling overseas.

The effect of the disease on trade may be contained because most of the birds are for the domestic Christmas market. Britain has now lost its avian flu infection-free status again, preventing all trade of poultry with countries outside the EU. Eggs and hatching birds from outside the area can be traded with EU countries.

Last night the Tories attacked the government for what they claimed were animal health budget cuts. Peter Ainsworth, shadow environment secretary, said: "It is astonishing that in a year which has seen foot and mouth disease, bluetongue and now a second outbreak of avian influenza, local animal health teams are being told to slash their budgets.

"Local councils were originally promised £9.77m under a scheme set up to fight animal disease, but only £8.5m was ever allocated."